Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 081754
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
1254 PM EST Fri Dec 8 2017

.SYNOPSIS...
Issued at 458 AM EST Fri Dec 8 2017

Today will be mostly sunny but cold with highs in the lower 30s.
Clouds will increase late this afternoon and evening as a clipper
system approaches the area. This system will bring light snow to
the entire area late tonight into Saturday morning. Most
locations will receive between 1 and 3 inches of snow with the
lower amounts south and east. This will transition to lake effect
snow across northwest Indiana and southwest Lower Michigan
Saturday afternoon into Saturday evening. Total accumulations in
the snow belt region are expected to be between 4 and 8 inches
with some locally higher amounts possible.

&&

.SHORT TERM...(Today and Tonight)
Issued at 458 AM EST Fri Dec 8 2017

...Winter Storm Watch upgraded to Warning for Lake Effect on
Saturday...

Subsidence and dry air has led to clear skies over entire area early
this morning. Expect mostly sunny start to the day but weak short
wave to our north may spread some high clouds into far north later
today. Despite sunshine it will remain cold with highs only in the
lower 30s.

The evening will begin dry but potent short wave will bring light
snow to most areas late tonight and into Saturday morning.
Isentropic lift expected to develop late this evening ahead of wave
with strong lift noted over our area for a brief period. Dry lower
levels and evaporation a concern initially and NAM12 seems to be
picking up on this with a more delayed start to onset of pcpn and
less QPF compared to previous runs. While moisture is limited and
low levels initially dry, mid level trough will be sharpening as
100kt jet streak dives south with left front quad providing good
diffluence aloft and aiding in synoptic scale lift. Most hires
models spread light snow across much of the area before 12z except
far southeast. Will lean toward these solutions for continuity.
Expect generally 1 to 2 inches late tonight with the 2 inches over
northwest areas where some lake enhancement will be kicking in
toward 12z Saturday.

&&

.LONG TERM...(Saturday through Thursday)
Issued at 458 AM EST Fri Dec 8 2017

Synoptic short wave trough will be pushing across the area Saturday
morning while lake enhanced snow becomes pure lake effect with
strong dominant band expected on Saturday. More cold air and lake
effect expected next week.

Strong PV anomaly still being advertised with this short wave as 1.5
PVU surface dropping as low as 700mb on Saturday as it crosses
our area. Strength of this system will be enough to squeeze out
what little moisture is available with overperformance possible
especially north and east with entrainment of lake moisture. Snow
ratios initially around 15:1 but trending toward 20:1 by afternoon
as colder air deepens. Synoptic snow totals still expected in the
1 to 3 inch range with the lower amounts south and higher amounts
north. Expect lingering snow showers away from the lake in the
afternoon with cold air advection and upper trough moving through.

Attention quickly turns to lake effect snow event for Saturday. As
mentioned...lake enhanced snow should begin as surface low drops
into western Michigan early Saturday morning and winds over the lake
become northwest then north. Delta Ts still climb into lower 20s
while theta e lapse rates around -2 k/km and inversion heights above
10kft. Models have been trending toward possible evolution of
several mesoscale vortices as trailing surface trough slides south
across lake in wake of surface low and lake/land convergence leads
to circulation development. Pattern fits nicely with locally
researched Type VI development which features a mesoscale low and
vort center with an attached single band in its wake. Several hires
models trying to show this with strong 925mb vort center dropping
south Saturday morning. If this mesovort does develop it will aid in
enhancing snowfall rates when it moves onshore but at same time will
lead to more of a meandering and wavy single band that may bend a
little further west than currently expected.  Composite reflectivity
trends from hires models still show most of the single band
remaining over southwest Berrien, Western St. Joseph and central
Laporte counties despite the meandering nature. Models are in good
agreement that warning level snow will fall across these 3 counties
which were in a watch. Have therefore upgraded to a warning. Amounts
will still depend on the evolution and movement of the dominant band
but given the favorable environment we have been discussing along
with long fetch and pre-conditioning should see intense snowfall
rates of 1 to 2 inches per hour. Total accums in the 4 to 8 inch
range with locally higher amounts if and where this band becomes
stationary.

Still concerned for Starke and Marshall counties getting
warning level conditions given fetch length and wind speeds pushing
heavy snow well inland as is often the case with these single bands.
The problem is the transitory nature of band this far inland. Did
put these counties into an advisory with 3 to 6 inches looking good
with isolated higher amounts possible. Opted for now to leave
Elkhart and Cass MI out of headlines with quick transition to Single
band Saturday morning likely limiting additional accumulations and
even a break in snowfall for a period during the day. These areas
will be monitored for possible inclusion into headlines.

Generally accepted blended solution for next synoptic system early
next week. Some warming noted in models on Monday but quick drop in
temps with evaporative cooling. Kept pcpn type as snow for now
though could be a mix depending on timing and location of pcpn.
Models have trended north with pcpn last few runs. Another arctic
surge behind this wave to keep temps below normal and good
possibility of another long fetch lake effect event. Increased Pops
in favored lake effect locations for much of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday Afternoon)
Issued at 1254 PM EST Fri Dec 8 2017

VFR conditions will prevail through 6Z Sat before the next system
begins to impact the area. Light snow will overspread both TAF
sites starting between 6 and 9Z at KSBN and 8 to 11Z at KFWA with
MVFR visibilities settling in at the respective sites. Given some
concerns as to how fast impacts will occur a downward trend is
noted, but nothing below 2 SM for this package. It is possible
that pockets of lower vsby and cigs could materialize with the
initial snow, but not worth addition of a tempo at this point.

Lake effect snowshowers will organize and likely impact KSBN at
some point after 15Z. Some concerns as to exactly how fast the
band develops and how long it resides over/near the airport.
Potential exists for much lower cigs/vsby with sig impacts at
times at KSBN if the main band settles over the airport. While
some impacts are possible from the band at KFWA, many more
variables warrant low confidence.


&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Weather Advisory from 4 AM EST /3 AM CST/ Saturday to
     midnight EST /11 PM CST/ Saturday night for INZ012-014.

     Winter Storm Warning from 1 AM EST /midnight CST/ Saturday to
     midnight EST /11 PM CST/ Saturday night for INZ003-004.

MI...Winter Storm Warning from 1 AM Saturday to midnight EST Saturday
     night for MIZ077.

OH...NONE.
LM...Small Craft Advisory until 10 AM EST Saturday for LMZ043-046.

     Gale Warning from 10 AM to 10 PM EST Saturday for LMZ043-046.

&&

$$

SYNOPSIS...MCD/Lashley
SHORT TERM...Lashley
LONG TERM...Lashley
AVIATION...Fisher


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